Answer:
3/4, -3/4
Step-by-step explanation:
Using the difference of squares we can change things around.
cos(a)^4 - sin(a)^4
(cos^2(a) + sin^2(a))((cos^2(a) - sin^2(a)) then sin^2 + cos^2 = 1
cos^2(a) - sin^2(a) Using the same identity sin^2 = 1 - cos^2
cos^2(a) - (1 - cos^2(a))
cos^2(a) - 1 + cos^2(a)
2*cos^2(a) - 1
So let's use this.
2*cos^2(a) - 1 = 1/8
2*cos^2(a) = 9/8
cos^2(a) = 9/16
cos(a) = +/- 3/4
so 3/4 can be positive or negative
